I now have managed to copy the songs3.db – so i should be able to copy it back if it happens again that my smart playlist should vanish again some day.

Fyi: i did the following:
the devellopers of the extensions on my NAS that made firefly available for my NAS had provided an option to create a symbolic link to the .zap/… – folders which made all the files and folders there visible for windows.
So that sqlite-firefox-plugin could easily find the database and it made a copy of the database before opening it.